1. Field of the Invention
The present invention relates to a spark plug of a type that is to be mounted on an internal combustion engine, and a producing method thereof.
2. Description of the Related Art
A glass sealable spark plug is conventionally known and it is manufactured by a process comprising the steps of: filling a seal glass material or a combination of seal glass material, a resistive material and seal glass material as a glass sealing material within a space between a center electrode at a top end thereof and a terminal electrode at a rear end thereof which are arranged in a axial hole of an insulator; melting the glass of the glass sealing material by heating; pressing the terminal electrode; and cooling them to solidify.
(A) When glass sealing is effected at a comparatively low temperature using glass sealing materials of low softening point, the energy cost is reduced and yet high operating efficiency is provided.
On the other hand, if the sealed portions (i.e., the top end of the terminal electrode and the rear end portion of the center electrode) are exposed to high temperature, the glass sealing materials will soften and both the terminal and center electrodes will loosen to impair the airtightness of the spark plug.
In addition, the binding force of the electrically conductive substance mixed in the sealing materials and the resistive material drops to produce a higher resistance.
(B) When glass sealing is effected at high temperature exceeding 800xc2x0 C. using glass sealing materials of high softening point exceeding 750xc2x0 C., the terminal electrode having a plate of nickel or zinc applied to a low carbon steel is oxidized to corrode in the process of glass sealing.
If the terminal electrode is oxidized to corrode, the plate will come off the steel to cause rust formation on the latter.
If rust forms, the electrical connection of the terminal electrode to the plug cap will deteriorate. In addition, the rust stains the barrel portion of the insulator to cause flashover.
It is an object of the invention to provide a spark plug that can be produced without causing oxidation and corrosion of the terminal electrode during glass sealing and which is protected against the increase in the resistance between the terminal and center electrodes.
A spark plug according to the present invention is comprised of an insulator having an axial hole; a center electrode provided at the top end side of the spark plug; a terminal electrode provided at the rear end side thereof, the center electrode and the terminal electrode being arranged to be opposite to each other in the axial hole; and seal glass filled in the axial hole between the center electrode and the terminal electrode; wherein glass sealing of the seal glass is effected at a temperature in the range of 500 to 1000xc2x0 C. at an oxygen concentration of not more than 12 vol %.